Adsorbents are employed in a wide variety of applications ranging from drying of bulk industrial gas streams to climate control by dehumidification, adsorptive cooling or heating such as in various heat pump and desiccant cooling schemes as well as selective separation of organic compounds from mixtures of the same with other organic compounds.
Nearly all such commercial processes are cyclic as presently practiced and involve an adsorption stage wherein a component or components is adsorbed from a gas stream, followed by desorption of the adsorbed material or materials to regenerate the adsorbent, which is then recycled into contact with an incoming gas stream for renewed adsorption.
Such processes are well known in the art and are described in numerous technical publications and patents. Typical of such, herein incorporated by reference, are U.S. Pat. Nos. 4,701,189; 3,844,737; 4,134,743; 4,197,095; 4,247,311; 4,783,201, 4,070,164, as well as D. W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, Inc., Chapter 8.
A particular difficulty with regard to gaseous separations is encountered when it is desirous to remove organic gases in the presence of water vapor such as moist air. Situations of this type are encountered in the so-called "sick building syndrome" when solvents from construction materials used to build large insulated buildings leach into the air causing potential health hazards and much social and political concern. In addition, there are other and more common causes where organic contamination of air represents a clear problem, including painting operations, cleaning operations or any other operations which involve the use of an organic solvent.
Presently available adsorbents have many disadvantages which limit their effectiveness in separating volatile organic compounds (VOC) from a mixture of the same with moist air.
By way of explanation, presently available adsorptive desiccants are of two general types, namely, classical crystalline zeolitic molecular sieves, such as zeolites Chabazite, A and X and amorphous (noncrystalline) inorganic metal oxides or metal silicates, such as silica gel or certain alumina gels or certain porous forms of carbon. Classical molecular sieves strongly and preferentially bind water by electrostatic interaction with water and the cations inherent to most zeolite structures.
Although classical molecular sieves also bind organic molecules, such binding is weaker than with polar compounds such as water. As a consequence, water rapidly "covers" all the active sites in such zeolites effectively blocking the adsorption of volatile organic compounds.
On the other hand, carbons will bind organics in preference to water. However, the binding is so strong that regeneration is expensive and degradation of the carbon adsorbent usually occurs.
If a balance could be reached where water and volatile organics could both be easily adsorbed and desorbed by the same absorbent, then systems conventionally used for dehumidifying air could be used for removing said volatile organics from moist air.